1. Field of the Invention
This invention relates to the field of digital sector servo systems for use in a magnetic disk drive.
2. Background Art
In a typical rotating medium as storage system, data is stored on magnetic or magneto optic disks in a series of concentric "tracks." These tracks are accessed by a read/write head that detects variations in a magnetic orientation of the disk surface.
The read/write head moves back and forth radially on the disk under control of a head-positioning servo mechanism so that it can be selectively positioned over a selected one of the tracks. Once in position over a track, the servo mechanism causes the head to trace a path which follows the centerline of the selected track. This maximizes head-to-track registration, and permits accurate recording and reproduction of data on the track.
In order to effectively operate the head positioning servo mechanism, it is necessary that the position of the head with respect to the tracks be known. In addition to knowing which track a head is over, it is necessary to know where on that particular track the head is positioned.
In the prior art, position information is provided through the use of special servo patterns that are reproduced by the recording heads in the drive. A servo pattern is a permanent pattern prerecorded on the storage disk at the time of assembly of the completed disk drive. The servo pattern is detected by the head, and after appropriate signal processing, yields track position information. One prior art method of providing servo position information is known as a "sector" servo method.
In the sector servo method, bursts of servo information are disposed on a disk surface in between data areas. Each servo burst contains track position information, track radial address fields and index information. Space-division multiplexing of data and servo information minimizes track misregistration effects, since both data and servo information are reproduced from the same surface with a common head. Thus, sector servo schemes are particularly useful for disk drives with high track density.
Once a track has been located, it is important that the read/write head be kept on the centerline of that track for accurate reading and writing operations. This positioning of the read/write head on the centerline of a track is known as "track following." Variations from the centerline of the track being followed produce position error signals (PES) which are used to generate a corrective input to the head positioning apparatus to move the head back to the centerline position.
One prior art method for detecting a position error signal using integration techniques in an analog servo system is disclosed in Herrington and Mueller, IBM Technical Disclosure Bulletin, Vol. 21, No. 2, July 1978, pages 804-805. The system of Herrington and Mueller has the disadvantage of being limited to a dedicated servo pattern in an analog servo system, and is intended for use with a continuous, as opposed to a sampled, pattern.
Another prior art demodulation method is disclosed in Commander and Taylor, Disk Storage Technology, IBM Publication GA26-1665-0, February 1980. Commander and Taylor's system uses a synchronously detected null-type sector servo pattern as a supplement to a dedicated servo pattern system, in what is termed a `hybrid` servo. The demodulation method relies on timing information extracted from the dedicated servo pattern and is, therefore, not directly applicable to a purely sector servo system. Further, the result of the demodulation process in the Commander and Taylor article is retained in analog form throughout the servo system and all AGC (automatic gain control) operations are performed in the analog domain.
Roalson, U.S. Pat. No. 4,551,776 discloses the use of a digital sequencer to obtain automatic calibration of AGC gain reference in a servo demodulator similar in construction to that of the Herrington and Mueller system. The reference does not suggest the applicability of the system to sample data use, and the AGC feedback loop is of entirely analog construction.
Betts, U.S. Pat. No. 4,511,938 discloses an extension of the elementary single phase null-type servo pattern to a multiplicity of phases. This system applies only to sector servos of the analog type. Furthermore, the circuits disclosed require use of a separate integrator for each phase of the multi-phase quadrature servo pattern. Moreover, the AGC system is complex and is essentially an analog loop with digitally selected reference input. Position signal processing is accomplished entirely by analog methods which are also complex. Additionally, in the Betts patent, the carrier required for signal detection is generated by establishing the phase of a constant frequency carrier source used in the demodulation process by a single isolated transition of the servo pattern. This method can deliver a misphased carrier if the initializing transition is corrupted by noise. Such corruption becomes increasingly likely as recording densities increase and signal-to-noise ratios fall.
Laishley, U.S. Pat. No. 4,297,734 illustrates the use of a null pattern and describes a detection method not readily extensible to accurate digital processing of the servo position error signal.
Therefore, it is an object of the present invention to provide a sector servo demodulation means providing, in digital form, position error signals representative of magnitude and sign of read/write head displacement from track centerline and suitable for direct application to a digital signal processor which implements the servo control algorithms.
It is another object of the present invention to provide a servo controller suitable for sector servo use, which is implemented in a digital signal processor.
It is yet another object of the present invention to provide PES detection by an integration method using only two integrators to demodulate a position error signal having an arbitrarily large number of phases.
It is still another object of the present invention to provide fully digital automatic gain control in the PES detection scheme.
It is a further object of the present invention to provide a means for generating a carrier signal required for synchronous detection of PES signals.